This invention relates to a fish-finder which detects a school of fish by emitting ultrasonic waves into the water and displaying the reflected waves on the screen of a cathode ray tube.
In conventional types of fish-finders, upon emission of ultrasonic waves, a recording pen, initially placed on a recording paper, is driven to run across the recording paper travelling in its lengthwise direction and received reflected waves are supplied to the recording pen to discolor the recording paper in accordance with the levels of the received reflected waves to record shaded images. With the method using such a recording paper, small differences of shading do not appear, so that the amount of information to be displayed is small. Where the level difference of the reflected waves is not large as, for example, in the case of fish in a shoal of plankton, the object to be detected cannot be distinguished from the surroundings. For the same reasons, it is impossible, of course, to judge the kind of fish from the records on the recording paper. Also, it is difficult to discern a school of fish close to the bottom of the water from the recorded images obtained on the recording paper. In the conventional fish-finders, the amount of information of the images recorded on the recording paper is remarkedly smaller than the information of the received reflected waves, as described above. Further, it is also difficult to magnify one part of the record and display it along with the non-magnified record. In some cases, it is desired to magnify one part of the record on the basis of, for example, the seabed. If related information, for example, the information from the fish-finder and that from a net monitor, are recorded side by side, the information can be analyzed more accurately. With the prior art, however, it is relatively difficult to record a variety of information at the same time. Further, it is more difficult to arrange the conventional fish-finder so that it may be freely switched for the operation of a plurality of signals and recording one of them over the entire width of the recording paper and for the operation of recording two or more of the signals in side-by-side relation. Moreover, it is difficult to adjust the relative positions of a plurality of parallel records, so that interrelationships of the records cannot be judged with ease. In the conventional fish-finders of the abovesaid type, recording papers are expendables and it is necessary to prepare a sufficient amount of recording paper at all times.
An object of this invention is to provide a fish-finder in which received reflected waves are displayed clearly in distinction from one another even if they have small level differences.
Another object of this invention is to provide a fish-finder in which a magnified display and a high-speed display can be easily provided.
Another object of this invention is to provide a fish-finder in which a variety of information displays such as a magnified display, a normal display, a display of information from a net monitor, etc. can be provided side by side and these displays can be selectively provided.
Another object of this invention is to provide a fish-finder in which a plurality of information can be displayed in parallel and the relative positions of the displays can be easily changed to facilitate an easy interpretation of their relationships.
Still another object of this invention is to provide a fish-finder which does not employ a recording paper and does not require any articles of consumption.